1. Field of the Invention
The present invention relates to a copier, facsimile apparatus, printer or similar image forming apparatus and more particularly to an image forming apparatus of the type transferring toner images to both surfaces of a recording medium with a so-called one-pass type of image transfer system.
2. Description of the Background Art
An image forming apparatus configured to transfer images to both surfaces of a sheet or similar recording medium is conventional and uses either one of two different image transfer systems, i.e., a switchback system and a one-pass system. The switchback system conveys a sheet via image transferring means to thereby transfer a toner image from a photoconductive drum or similar image carrier to one surface of the sheet, switches back the sheet, and then transfers another toner image to the other surface of the sheet. On the other hand, the one-pass system transfers toner images to both sides of a sheet with image transferring means substantially at the same time without switching back the sheet. The one-pass system is advantageous over the switchback system in that it is free from an increase in cost and an increase in image forming time ascribable to a sophisticated switchback mechanism.
An image forming apparatus using the one-pass system stated above is disclosed in, e.g., Japanese Patent No. 2,906,538 and Japanese Patent Laid-Open Publication No. 2000-105513. The apparatus taught in U.S. Patent No. 2,906,538 includes switching means for switching the polarity of a second toner image developed on a photoconductive element and passes a sheet between the photoconductive element and an intermediate image transfer body to thereby form toner images on both sides of the sheet at the same time. On the other hand, the apparatus taught in Laid-Open Publication No. 2000-105513 includes two developing units each for developing a latent image with a developer of particular polarity and transfers toner images formed by the two developing units to both sides of a sheet.
A conventional one-pass type of image forming apparatus includes a first and a second intermediate image transfer belt, as will be described specifically later with reference to FIG. 1. Toner images of different colors formed on, e.g., photoconductive drums are sequentially transferred to the first intermediate image transfer belt one above the other, completing a color image. The color image is then transferred to the second intermediate image transfer belt. A charger or tertiary image transferring means adjoins the second intermediate image transfer belt for transferring the toner image from the belt to a sheet.
In the one-pass type of apparatus stated above, the image carried on the second intermediate image transfer belt, but not transferred to a sheet, is conveyed to a nip between the belt and the tertiary image transferring means. In addition, the image transferred to the sheet, but not fixed, is also conveyed to the above nip. Therefore, the tertiary image transferring means should not be implemented by a transfer roller contacting the second intermediate image transfer belt, but should be implemented by a charger not contacting the belt.
The problem with a charger is that it produces ozone and other toxic discharge products during operation. Further, in the above conventional apparatus, tertiary image transfer is effected with toner existing on a sheet with the result that the toner is scattered in accordance with the electric field of discharge and contaminates the charger.
In the apparatus disclosed in U.S. Pat. No. 2,906,538 or Laid-Open Publication No. 2000-105513 mentioned earlier, it is possible to obviate discharge products and toner scattering ascribable to a charger by omitting the tertiary image transferring means. However, switching the polarity of a toner image formed on a photoconductive element as in U.S. Pat. No. 2,906,538 or using two developing units as in Laid-Open Publication No. 2000-105513 results in a sophisticated configuration and an increase in cost. In this respect, neither one of such prior art technologies is a drastic solution to the problems particular to a charger.
Further, the polarity switching means proposed by U.S. Pat. No. 2,906,538 is implemented by a corona charger that also produces ozone and other toxic discharge products.
Technologies relating to the present invention are also disclosed in, e.g., Japanese Patent Laid-Open Publication No. 3-253881.